A method called mass spectroscopy is capable of identifying a sample by measuring the ratio of mass to electrical charge (mass-to-charge ratio: m/z) in a sample enveloped within electrical charges such as ions within a magnetic field. A typical device and method for mass spectroscopy currently in wide use is the ion trap which captures ions within a trap made up of electrodes and then selectively emits ions by changing the electrical potential within the trap.
An ion trap called an RF (radio frequency) ion trap may for example utilize a Paul Trap constituted by one doughnut-shaped electrode (called a ring electrode) enclosed by two bowl-shaped electrodes (called end caps) to focus ions at one point in the center of a ring electrode by applying a radio frequency voltage to that ring electrode. (Refer for example, to U.S. Pat. No. 2,939,952, Quadruple Storage Mass Spectrometry: R. E. March and R. J. Hughes, John Wiley and Sons ISBN 0-471-85794-7, Quadruple Ion Trap Mass Spectrometry: Raymond E. March and John F. Todd, Wiley-Interscience ISBN 0-471-488887). This ion trap focuses the ions spatially in three dimensions within an electric field and is therefore sometimes called a three dimensional trap.
A linear ion trap is formed from four rod electrodes arrayed in parallel in a quadruple state, and traps ion in a center region made up by the four rods by applying a radio frequency voltage between the two facing electrode pairs. This ion trap is also called a two dimensional ion trap because the ions are focused in two directions by the radio frequency.
There is also a method for trapping charged particles around a center electrode (Refer for example to JP-A-Hei9(1997)-61597) by overlapping a direct current field and an alternating current field and applying them to a space formed by a center electrode, and an external electrode made up of quadrupole rods.